This invention pertains to an improved coal gasification process and apparatus using a fast fluidized bed gasifier assembly for producing low- and medium-Btu gas products. It pertains particularly to such gasification process wherein fine secondary char is returned to the gasifier's lower end.
The gasification of hydrocarbon solids such as coal or coke and residual oils using a fast fluidized bed of char particles at high superficial velocities and moderately high density has been previously proposed, as described in U.S. Pat. Nos. 3,840,353; 3,957,457 and 4,032,305 to Squires. Fast fluid beds are those fluidized beds operating in a fast fluidization contacting regime, and have characteristics of high superficial gas velocity, i.e.; five to ten fold higher than normal or conventional fluidized beds, and high solids circulation rates along with a high degree of solids backmixing. The fluidized bed density for a given solid is a function of the solids circulation rate as well as gas superficial velocity, and provides high heat and mass transfer rates. Steam and oxygen-containing gas are introduced into the lower portion of the bed to provide the fluidization and the reactants needed for gasification.
In previous experimental work on coal gasification using a fast fluidized bed process, fine char material from a secondary cyclone separator was removed but not recycled to the gasifier. Results indicated that less than about 50% of the carbon in the feed coal was converted to gases and liquid products using such a fast fluid bed gasifier configuration. Thus, improvements in char utilization for such coal gasification processes are clearly needed to produce higher conversion of carbon and improved yields of fuel gas products having low to medium heating values.